Harker School senior Arjun Mehta, whose idea as a 12-year-old turned into a business that Visa bought for $200 million, is back with a new startup.

Mehta and two classmates — Divyahans Gupta and Simar Mangat — have been working for about three years on Stoodle. It is a whiteboard and voice conferencing app that students can use to help each other on homework and to collaborate on projects.

The growth of Stoodle is the type of story that would be highly unlikely anywhere but in Silicon Valley.

It starts with Mehta, who is the son of Karl Mehta, an entrepreneur now working as a venture partner at Menlo Ventures. The elder Mehta built an idea his son came up with into PlaySpan — a marketplace where online gamers could sell virtual goods for real money. After a few years of building it up, the Visa sale provided a healthy exit.

A few years later, Arjun transferred from the public schools in Fremont to the private Harker School in San Jose.

"Right away I noticed a big difference in the quality of education and access to resources that kids at Harker have," Mehta told me. "I started to wonder what would happen if there was a way for students from my old school to get help from students at my new school."

Then influential venture capitalist Vinod Khosla came to Harker to give a keynote at a student conference. Mehta seized the opportunity to pitch Khosla, who liked what he heard and referred the teen to his wife, Neeru.

"I was immediately taken by the passion and commitment and intelligence of the Stoodle team," Neeru Khosla, the head of the education-oriented CK-12 Foundation, told me. The foundation provided Stoodle with the funds needed to take their business to the cloud.

Stoodle is now being used by a few thousand students and teachers, some from as far away as Mississippi, but Mehta and team are recruiting evangelists to promote the product around the country.

"There are other products people use for this but they are either based on Flash, which limits how it can be used, or they aren't designed for student-to-student communication, like our is," Mehta said.

Stoodle will always be free for students but it might get revenue in the future from schools, Mehta said, although that future isn't clear beyond June when he and Gupta graduate. The other partner, Mangat, is already a freshman at Stanford.

Neeru Khosla said that CK-12 Foundation will continue to support Stoodle as long as it is being actively used and Mehta said the founders would be happy to maybe pass it along when they all move on to college.